1. Field of the Invention
The present invention relates to a zoom lens, and more particularly, to a zoom photographic lens suitable for use in an image pickup apparatus such as a video camera, a monitoring camera, a digital still camera, a camera for broadcasting, and a silver-halide film camera.
2. Description of the Related Art
In an image pickup apparatus such as a video camera, a monitoring camera, and a digital still camera based on a solid-state image sensor, it is desirable a zoom lens having a wide angle of view and a high zoom ratio over the entire zoom range. In addition, as a high-definition quality of solid-state image sensors increases, the zoom lens is demanded to have high optical performance accordingly.
A four-unit zoom lens including first to fourth lens units having positive, negative, positive, and positive refractive powers in order from an object side to an image side is currently known. In a rear focus type zoom lens in which magnification is varied by moving the second lens unit, and a variation of the image plane caused by the variation of magnification is corrected and focusing is performed by moving the fourth lens unit.
In order to obtain a high zoom ratio as well as miniaturization of the entire zoom lens, it is desirable to increase the refractive power of each lens unit of the zoom lens. However, if the refractive power of each lens unit increases, a variation of aberration caused by zooming also increases. Therefore, it is difficult to obtain excellent optical performance over the entire zoom range.
In addition, as the zoom ratio increases, chromatic aberration or curvature of field increases over the entire zoom range from the wide-angle end to the telephoto end. Thus, it is difficult to appropriately correct chromatic aberration or field curvature over the entire zoom range. In order to miniaturize the entire zoom lens with a wide angle of view and a high zoom ratio in the rear focus type four-unit zoom lens described above, it is important to appropriately arrange the refractive power of each lens unit and a lens configuration of the first and second lens units. If such a configuration is not appropriate, variations of various aberrations caused by zooming significantly increase, so that it is difficult to obtain miniaturization of the entire zoom lens with a predetermined zoom ratio and a high optical performance.